Groundwater is Vitens’ main source for the production of drinking water. Most of Vitens’ production sites have at least one sand filtration step for the removal of iron, manganese and other unwanted substances in drinking water. After a while, the filters have to be backwashed. In the past, all the backwash water used to be buffered and after clarification discharged to the surface water. In 1998, Vitens started reusing backwash water at its Fledite production site.
Vitens applies three criteria for reusing backwash water at a production site:
- reuse is economically feasible
- few alternative sources
- authority sets (quantitative) requirements on discharges
In the past, several pilot studies were conducted relating to backwash water reuse. Multiple options were explored varying from plate separator to various membrane systems. These studies resulted in a Vitens standard for reusing backwash water: ultra filtration (submerged) combined with UV disinfection. The combination of robustness and a double microbiological barrier were decisive in this.
The pilot studies also showed that after ultrafiltration, some of the manganese was not rejected by UF and was present in the permeate. It is therefore necessary to use a filter before it meets our desired quality. The process scheme for backwash water reuse at Vitens looks like this:
The backwash water is reused at five Vitens sites: Fledite (reuse since 1998), Spannenburg (2001), Noordbergum (2005), Oldeholtpade (2006) and Engelse Werk (2010). A new site was added in 2013: Wierden.
For the Wierden site, six UF/MF were tested in a parallel pilot study. The membranes varied in pore size, membrane material (ceramic and polymer), configuration (spiral wound, capillary, tubular) submerged and pressurised systems. Three similar priced systems met our requirements. due to the limited space in the existing building at wierden, we opted for the system with the smallest footprint microfiltration with ceramic membranes.